SiC semiconductors utilizing SiC single-crystal are viewed as promising candidates for next-generation power devices to replace Si transistors. Applications as substrates for blue light-emitting diodes and laser diodes are also expected.
In addition to being physically and chemically stable, SiC single-crystal is a material that can withstand high temperature and radiation. It is therefore thought to have potential for application as an environment-resistant semiconductor material. Moreover, SiC power devices are also attracting attention, particularly as energy-saving devices, because the power loss in the device can be reduced greatly in comparison with that in a conventional Si device.
However, no crystal growth technology has yet been established that can provide a stable supply of high-quality SiC single crystal of large surface area on an industrial scale. SiC has therefore not been put to extensive practical use in spite of being a semiconductor material having the numerous merits and high potential explained in the foregoing.
As a currently available method for obtaining high-quality SiC single crystal of relatively large diameter, there is known the modified Lely process, which conducts sublimation recrystallization using an SiC single-crystal substrate as a seed (Yu. M. Tairov and V. F. Tsvetkov, Journal of Growth, vol. 52 (1981) pp. 146-150). Owing to its use of a seed crystal, this process can control the crystal nucleation process and, by controlling the ambient inert gas pressure, can control crystal growth rate with good reproducibility. The modified Lely process makes it possible to grow SiC single crystal while controlling its polytype (6H, 4H etc.), carrier type and concentration. Currently, 2-inch (about 50 mm) to 3-inch (about 75 mm) SiC single-crystal wafers are being produced by the process, processed into substrates and subjected to epitaxial thin film growth and device fabrication.
However, current commercially available SiC single-crystal substrates have many quality problems, and crystal quality improvement is essential for making SiC single crystal and SiC single crystal devices suitable for practical use in the future.
Elimination of the leak current and other unstable characteristics arising in the SiC semiconductor is indispensable for realizing high-performance SiC power devices with stable characteristics. The cause of the unstable characteristics is thought to be related to the quality of the SiC single-crystal substrate. The degradation of the quality is thought to be caused mainly by crystallographic defects formed in the substrate, namely, micropipe defects, dislocation defects and the like. Various efforts have been made regarding micropipe defects, particularly with regard to reducing them, and fabrication of substrates with no micropipes has been reported to be possible. However, methods available for reducing dislocation defects are almost all ones that rely on special processes such as converting the growth crystal surface, as taught by, for example, Japanese Patent Publication (A) Nos. H5-262599, H8-143396, and 2003-119097, and no simple industrially effective means is known.
Japanese Patent Publication (A) No. H5-262599 teaches a method for obtaining good quality SiC single crystal. Specifically, it teaches that in a sublimation recrystallization process for growing SiC single crystal on a seed crystal of an SiC single-crystal substrate, it is effective to use as the seed crystal one composed of SiC single crystal in which a crystal plane at an offset angle of about 60° to about 120° from the {0001} plane is exposed. Japanese Patent Publication (A) No. H8-143396 teaches a method in which a first SiC single crystal is grown using as a first seed crystal a crystal plane of an SiC single crystal inclined about 60° to about 120° from the {0001} plane, a wafer having a {0001} plane is newly taken from the first SiC single crystal, and crystal is grown using the new wafer as a second seed crystal. Japanese Patent Publication (A) No. 2003-119097 teaches a method of producing low-defect crystal. In each of a first growth process, (n−1)th growth process, and nth growth process, a different crystal plane is cut, and cut as a seed crystal that is grown.
In the case of producing a final device, the general practice is to use a plane near the {0001} plane, and in the sublimation recrystallization processes, which fundamentally always grow crystal on a seed crystal, it is considered that the methods, which all cut a different plane, grow it as a seed, and again conduct final growth on the {0001} plane, are unsuitable as processes intended for mass production.
As a result, the situation today is that the market is being supplied with almost no products of a dislocation density lower than 8,000/cm2.
Moreover, although device properties are thought to be affected by dislocation pairs, dislocation rows and the like formed during growth near the {0001} plane by convergence of dislocations believed to be formed at high temperature owing to interaction between displacements, no effective method of suppressing them has been reported.
Evaluation of SiC single crystal is set out in The Institute of Electrical Engineers of Japan, Electronic Materials Research Group, Document No. EFM-88-24, p. 24, Sep. 5, 1988. According to this reference, examination of a crystal for defects by molten KOH etching causes many etch pits to occur in correspondence to the dislocations.